Robert pockets million: tidy sum (4)
I believe the answer is:
bomb
'tidy sum' is the definition.
The answer and definition can be both man-made objects as well as being singular nouns.
Maybe you can see a link between them that I can't see?
'robert pockets million' is the wordplay.
'robert' becomes 'bob' (short for Robert).
'pockets' indicates putting letters inside.
'million' becomes 'm' (abbreviation).
'bob' enclosing 'm' is 'BOMB'.
Can you help me to learn more?
(Other definitions for bomb that I've seen before include "One going off" , "Explosive device" , "Big bucks - flop" , "See 18" , "fortune" .)